Polyethers, chelating

Defore 1964 there was extensive literature on the effects of monofunc- tional Lewis bases on the reactivity of organolithium compounds, particularly in the polymerization field (1). However no information was available on the effects of chelating bases. Since chelating polyethers were in wide use, one can only speculate that they had been tried and were discarded because they rapidly decomposed the alkyllithium. 

See Antibiotics, polyethers Antibiotics, peptides Chelating agents. 

See Antibiotics, polyethers Catalysis, phase-transfer Chelating agents. 

Some of these compounds could be considered as dietary additives, but various other terms, including pesticides, can also be used. They can have beneficial effects on the environment and this aspect will be discussed later. The ionophore monensin, which is an alicyclic polyether (Figure 1), is a secondary metabolite of Streptomyces and aids the prevention of coccidiosis in poultry. Monensin is used as a growth promoter in cattle and also to decrease methane production, but it is toxic to equine animals. " Its ability to act as an ionophore is dependent on its cyclic chelating effect on metal ions. ° The hormones bovine somatotropin (BST) and porcine somatotropin (PST), both of which are polypeptides, occur naturally in lactating cattle and pigs, respectively, but can also be produced synthetically using recombinant DNA methods and administered to such animals in order to increase milk yields and lean meat production. "... 

Complexes with chelating organic reagents such as salicylaldehyde and -diketonales were first prepared by N. V. Sidgwick and his students in 1925, and many more have since been characterized, Stability, as measured by equilibrium formation conslanis, is rather low and almost invariably decreases in the sequence Lj > Na > K. This situation changed drainalically in 1967 w hen C. J. Pedersen announced the synthesis of several macrocyclie polyethers which were shown to form stable complexes with... 

Macrocyclic compounds with ion-chelating properties occur naturally and often function as ionophores, translocating ions across biological membranes many of these compounds are small cyclic polypeptides. Some natural carboxylic polyethers are selective for Li+ and are, therefore, ionophores for Li+. Monensin, shown in Figure Id, is a natural ionophore for Na+ but it will also complex with Li+ and it has been shown to mediate the transport of Li+ across phospholipid bilayers [21]. It has been proposed that synthetic Li+-specific ionophores have a potential role as adjuvants in lithium therapy, the aim being to reduce the amount of... 

Cyclic peptides, self-assembly of, 24 59-60 Cyclic peroxides, 18 436, 447-448, 459 Cyclic poly(aliphatic disulfide)s, 23 712 Cyclic poly(disulfide)s, available information related to, 23 713 Cyclic polyethers, 12 658 chelating agents, 5 710 Cyclic reforming operations, 25 166 Cyclic sesquiterpenoids, biosynthetic routes to, 24 472 Cyclic siloxanes... 

A class of macrocyclic polyethers built on a triazine ring (192) have been proposed in 1981 by Au.291 These compounds are active as phase transfer catalysts in the conversion of C8H,7Br to C8H17OPh. These molecules have chelating arms properly arranged in order to trap cations. 

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

Secondly, a calix[4]arene with four polyether substituents prepared by the Cragg group demonstrated no K+ transport but significant Na+ transport [55], Here, as with the Gokel group s more complex molecule described above, the calixarene was in the 1,3-alt conformation so that at best the cation could only bind to two -OCH2CH2O- chelation sites at any time. This would allow two water molecules to bind the cation giving a sixfold coordination centre ideal for Na+. Consequently 1,3-alt calix[4]arenes with simple polyether substituents appear to be selective for Na+ over K+. 

Pedulli and co-workers have also studied anion-radicals in the thiazole system, including those from 2-nitrothiazole, 2,2 -bithiazolyl, 2-arylthiazoles, and bis(2-thiazolyl) ketone. Here the anion-radicals were generated by alkali-metal reduction in ethereal solvents and found to exist in contact ion-pairs. Whenever the organic component contained two adjacent binding sites, chelation of the counterion occurred and was revealed by examining the variation of its hyperfine splitting with temperature and solvent. The interaction of the ion-pairs with macrocyclic polyethers was also studied by ESR for certain of the chelating radicals. ... 

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